Do you know how your touch screen works?

In this episode, you’ll learn about an important electrical effect called capacitance that can be used like batteries, to control timing, and even to detect touches.

Capacitance is the ability to store a charge. Not like how a battery produces charge through a chemical reaction but by storing positive and negative charge directly.

Let’s say you have a piece of metal. You can push extra electrons into the metal or draw electrons out creating either a negative or positive charge. But at some point it gets harder to move the charge. Think of an empty bus that you try to fill with people. At first, it’s easy for people to board but soon the bus becomes crowded and people start resisting other people getting too close. It doesn’t matter what shape the bus is just like it doesn’t matter what shape the metal is.

You can fit more people on the bus though if they all cooperate and don’t mind being so close. But what can you do about electrons?

A positive or negative charge creates a static electric field which will attract opposite charge just like how opposite magnetic fields attract each other. We can use this to pack electrons tighter and to remove more electrons.

All we need to do is fill one piece of metal with positive charge and another with negative charge. Then to get more charge, put the two pieces of metal close together but not touching so that the static electric field can reach into the other piece of metal and help pull more of the opposite charge into the other side. Both sides will end up helping fill the other side with charge.

But there’s only so much that an electric field can help and it gets weaker the farther away you go. Because the shape doesn’t matter, we can make the pieces of metal flat so more area can be placed next to the other side. Because the sides are flat, they’re called plates.

That’s all that really goes into making a capacitor. I mentioned that the sides shouldn’t touch because if they do, then the stored charge will equalize between the two sides and maybe ruin the capacitor. There’s a separator in capacitors called the dielectric. It’s really just a material that can stop direct current from flowing while letting electric fields pass easily. This material can even be air.

Capacitance is measured in farads and a one farad capacitor will be able to store one coulomb of charge and produce one volt between the two plates. Most capacitors are much smaller than a farad and are measured in microfarads which are millionths of a farad or picofarads which are billionths of a farad. But it is possible, especially with modern manufacturing to make large capacitors of a farad or more.

Listen to the full episode for more explanation about how capacitance can be used to detect when and where a person touches a screen.